Suntanning is perceived to enhance personal appearance. As a result, many individuals actively seek a tan by exposing themselves to the sun. Such exposure, as is well known from human experience, often results in painful erythema or sunburn. It may also lead to premature aging of the skin, certain types of dermatological diseases and even skin cancer.
The art has, therefore, long sought chemical agents which will color the skin the characteristic bronze color of a natural tan, but will do so without exposure, or with minimum exposure to the sun. Such agents must be easily available, inexpensive, non-toxic, stable on storage and capable of imparting an even, long lasting coloration to the human skin.
One of the first agents found to be effective in generating a tan coloration to the human skin was dihydroxy acetone. Unfortunately, this material was found to be more reactive with some areas of the skin than others resulting in a speckled appearance after use, and frequently in an unpleasant yellow color on the skin.
Tanning is believed to be the result of melanin formation, a reaction which is triggered by the action of sunlight, principally the UV component of sunlight.
The actual mechanism by which melanin forms in the human body is not a part of this invention. It is believed that melanin is an oligomer containing several indole segments formed as a result of the cyclization and polymerization of dihydroxy phenylalanine (DOPA) caused by exposure to the sun.
Not surprisingly, the art has turned to indoles as of possible utility in coloring the skin. It has been shown that a number of indoles can impart color to the skin. European Patent Publication 0,239,896, incorporated by reference herein is one example of coloration by treatment with indoles.
The main problem associated with the use of indoles as tanning enhancers is that they bring about the color change very slowly. Thus, it takes several days after the application of the indole to the skin before the skin achieves an acceptable coloration. This period of time is longer than that needed for the skin to sunburn.
In order to obtain more rapid coloration the art has turned to the use of metallic salts such as copper or ferrous sulfates in combination with indole. Unfortunately, these salts cause such a rapid formation of melanin that only the outermost surface of the skin is colored and the resulting tan is too easily removed by ordinary washings. In addition, the resultant color is often grey rather than tan, contributing to an unpleasant yellow hue of the skin.
The need, then, is for an agent which will color the skin sufficiently in an acceptable period of time, i.e. before it has a chance to sunburn and to a depth which is adequate so that the resulting tan will not be eroded by ordinary day to day activities.